You should have gone w/ the 19". The 4 blade is going to give you more
acceleration and hole shot. Top end will be hurt a little. The diameter of
a 4 blade is smaller than the 3 blade to compensate for the extra blade. In
theory your rpms would not change more than 50 or so as you say.
Your current speed should be about: 19 x 5900 / 1056 / 2 x .85 = 45 MPH
Your new speed will be about: 17 x 6150 / 1056 / 2 x .85 = 42 MPH
If you had stayed w/ the 19 you would be at 19 x 5850 / 1056 / 2 x .85 = 44
MPH

The 17 will give you better acceleration but you will need to keep the
engine under 6000 rpms for any extended period of time. Any way you can
change the order to a 19?

By the way - you should never run the engine wide open for extended periods
of time. You would not run your car in second gear at 6000 rpms down the
interstate. Don't do it on the river w/ your boat.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com

-
wrote in message
oups.com...
I have a Mercury 90 HP Four stroke with a max RPM (WOT): 5000-6000

With my default 19" black max (3 blade) RPM wide open is about 5900
RPM. I tested with a 17" black max and it did 6200 rpm.

I expect my newly ordered Trophy Plus 17" (4 blade) to do about 6150
RPM. Is this amount of revs a problem for this engine, or should I be
gentle with my right hand thus keeping it under 6000?

(following question comes to mind: how accurate is my factory fitted
mercury rev meter?)